The present invention relates to a bidirectional DC-AC inverter, and, more particularly, to a bidirectional DC-AC inverter that converts a direct current voltage input from a battery of a voltage higher than the voltage of a system power supply to an alternating voltage equal to the voltage of the system power supply and converts the alternating voltage input from the system power supply to a direct current voltage with which the battery is charged.
A hybrid car that is provided with an engine (internal combustion engine) has been applied to a practical use. In the hybrid car, drive wheels are driven by a motor at starting or in a lower speed range and the drive wheels are driven by the engine in middle and higher speed ranges to reduce fuel consumption and exhaust gas. Recently, a plug-in hybrid car that is provided with a battery that can be charged by a system power supply or a domestic power supply has been proposed to further reduce environmental load. For example, when a motor is driven by a battery that is charged with midnight power, a car can run a longer distance in an electric vehicle mode and the ratio of using electric power is increased compared to the ratio of using gasoline or other fuels. Therefore, compared to a general hybrid car, it is expected to reduce an emission of carbon dioxide and suppress air pollution. Since the system power supply reduces cost compared to generating electric power separately, a fuel cost can be reduced if a battery is charged with low-cost midnight power.
The system power supply outputs a sinusoidal alternating voltage and a sinusoidal alternating current. A charger used to charge a battery includes a capacitor input type rectifier circuit. An alternating current output from the rectifier circuit is, as illustrated in FIG. 5A, a harmonic current that flows only in the proximity of the peak of the alternating voltage provided by the system power supply. To improve power factor and suppress harmonic currents, a PFC (power factor control) circuit must be connected to the rectifier circuit. With reference to FIG. 5B, the PFC circuit converts an alternating current (a harmonic current) output from the rectifier circuit to a sinusoidal current waveform the phase of which coincides with the phase of the alternating voltage.
Japanese Laid-Open Patent Publication No. 11-356051 discloses a simply configured power supply device that increases efficiency of improving the power factor. As illustrated in FIG. 6, the power supply device includes a power factor improving circuit 71 and a memory 72. The power factor improving circuit 71 operates in such a manner that the phase of an alternating voltage and the phase of an alternating current input from an alternating current power supply AC (a system power supply) coincide with each other. The memory 72 stores information on a reference waveform. The power factor improving circuit 71 has diodes D1, D2, D3, D4, a reactor L, switching elements S1, S2, and a smoothing capacitor C. The diodes D1 to D4 rectify the alternating voltage and the alternating current input from the alternating current power supply AC. The reactor L and the switching elements S1, S2 forcibly cause the alternating current power supply AC to input an electric current. The smoothing capacitor C provides a smoothed direct current output. The power supply device includes an electric current detection circuit 73, a voltage detection circuit 74, a zero cross detection circuit 75, and a power factor control section 76. The electric current detection circuit 73 detects an input current. The voltage detection circuit 74 detects an output voltage. The zero cross detection circuit 75 detects a point of time when the polarity of the alternating voltage changes. The power factor control section 76 controls the power factor improving circuit 71. The power factor control section 76 includes a determining section 77 and a PWM control section 78. The determining section 77 determines a duty cycle by which the switching elements S1, S2 are subjected to PWM control. The PWM control section 78 outputs control signals in correspondence with which the switching elements S1, S2 are selectively turned on and off by the determined duty cycle. Specifically, the power factor control section 76 selectively turns on and off the switching elements S1, S2 by the duty cycle determined based on an input current value, an output voltage value, and the value of the reference waveform read out from the memory 72 with reference to the time point when the polarity of the alternating voltage changes. The power supply device improves the power factor by adjusting the duty cycle of the switching elements S1, S2.
Japanese Laid-Open Patent Publication No. 2003-143772 discloses an uninterruptible power supply device, which operates in the following manner. Specifically, if a power failure occurs, the device quickly switches the power supply to a load from a commercial alternating current power supply (a system power supply) to an inverter, without causing a short circuit between the commercial alternating current power supply and the inverter. The device also prevents a magnetizing inrush current from flowing through the inverter. The inverter is a bidirectional inverter that operates to charge a battery in a normal state and, if the voltage of the commercial power supply drops or the power is interrupted, converts a direct current output from the battery to an alternating current to supply the alternating current to the load. The inverter includes an H bridge circuit including four switching elements. When charging the battery, two of the four switching elements are turned off and the other two are selectively switched on and off.
The rated voltage of a traveling motor mounted in a plug-in type hybrid vehicle is higher (for example, several hundreds of volts) than the voltage of the system power supply. Thus, use of a battery with a rated voltage higher than the voltage of the system power supply improves the efficiency by which the power of the battery is consumed. In this case, the voltage output from the system power supply must be raised when the battery is charged by the system power supply. However, if two circuits including a booster circuit, which raises the voltage from the system power supply, and a PFC circuit are provided separately from each other, a reactor (a coil) for the booster circuit and a coil for the PFC circuit must be employed separately from each other. This increases the size of the device and raises the manufacturing costs.